2 Answers
2

Machatzit Hashekel Siman 262: We don't know when they are going to leave. So we ask them to bless us whenever they decide to leave. Similar to "Blessed are you when coming and blessed when leaving".

Avnei Nezer: Shalom Aleichem we say to the Malhachei Hashabat that "came" with us from Shul and Tsetchem Leshalom we say to the Malhacei Hachol (Mundane? weekday?) which were with us during the week. Similar to Ya'akov who had 2 groups of Malachim: One for Eretz Yisrael and the second for Chutz Laharetz.

Maybe that in the case of Ya'akov the 2 groups couldn't be together (those from Yisrael couldn't be abroad and vice-versa) but if I understand correctly according to the Avnei Nezer the pair that came with us will stay for the rest of the week so there is no problem in saying it in this order.
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ronyNov 12 '11 at 20:22

Although it seems strange to bid farewell to the angels so quickly -- why not invite them to join the Shabbat meal? -- Rabbi Yosef Yitzchak of Lubavitch explained that it is in bad taste to eat while others who are not eating (or in this case, cannot eat) are watching

I also once heard that the Lubavitcher Rebbe said that every second of an angel's existence is very precious and the angel doesn't want to waste time after it has given it's blessing, so we bid it farewell right away.